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Simsboro marks the culmination of a major pro gradational or regressive phase which began in the lower Hooper Formation. Note the multistacking of coarsening-upward, progradational delta-front sequences in the Hooper (fig. 15). The thin and discontinuous nature of the Hooper lignites is readily explained in terms of accumulation as interdistributary peats during active deltation or progradation. Following Simsboro deposition sedi mentation slowed and on foundering older deltas, extensive lignites accumulated as delta-plain blanket peats during lower Calvert Bluff deposi tion. These are the lignites being strip-mined at Big Brown near Fairfield and Alcoa in Milam County. The sedimentology of the remainder of the Calvert Bluff is not clear. In the Bastrop County and Leon County areas the lignite sequence (delta-plain deposits) appears, based on electric-log patterns, to be overlain by distributary channel deposits and/or
delta-front sequences (fig. 15). The latter signal the start in the upper Calvert Bluff of a second major progradational phase. Deltaic deposition closes the Wilcox which is, in turn, in outcrop uncon formably overlain by braided stream deposits of the Carrizo Sand (Claiborne Group).
Potential lignite deposits (table 3) are shown on figure 7. They have been outlined with regard to inferred stratigraphic occurrence in the lower Calvert Bluff Formation, reported outcrop and well occurrences (Dumble, 1892; Fisher, 1963), and projection from deep-basin occurrences. The value of the latter as a clue to near-surface occurrences is well illustrated in figure 7. Note the fingerlike projection in eastern Bastrop County pointing updip to the Sayersville-Bastrop district, an important old mining district. Another similar projection in western Houston County points
Table 2 (continued)—
County
Group or Formation
i Location
Thickness 1 (feet)
Facies
Panola
Wilcox
northwestern part; Beckville area (site of Industrial Generating Co. strip mine)
5-15
fluvial
Rains
upper Wilcox
southeastern part; vicinity of Ginger
3-10
fluvial
Robertson
Calvert Bluff
west-central part north of Calvert in general
3-12
deltaic
area of Brazos River; Little Brazos River and
Walnut Creek
Shelby
Wilcox
vicinity of Timpson and Stockman
4-6.5
lagoonal
Titus
Wilcox
southern half; vicinity of Winfield (site of
4-11
fluvial
Industrial Generating Co. strip mine),
Mt. Pleasant, CookviHe
Uvalde
Indio
southeastern part; east of Leona River to
5-10
lagoonal
west of Nueces River
Van Zandt
Wilcox
Canton and Edgewood areas
5-12
fluvial
Wood
Wilcox
extreme western edge; Alba-Hoyt district
8-13
fluvial
Zavala
Indio
northwestern part; astride the Nueces River
3-10
lagoonal
